Disclosed in US2006/0153612A1 is a conventional sheet processing apparatus equipped with a saddle stitch stapler, in which sheets discharged after a completion of operations required in an image forming apparatus are subjected to a booklet folding process by means of a booklet folding apparatus, booklet-folded sheets are placed sequentially on a saddle-shaped sheet stacking portion, positions of the fold lines of sheets constituting a bundle of sheets are aligned, staples are driven at a center portion of the aligned fold line to perform stapling by means of a stapling unit having a two-divided structure including a staple receiving mechanism and a driving mechanism, edge of the bundle of sheets is cut, and then the bundle of sheets is discharged.
However, in the above-mentioned apparatus, a saddle-shaped sheet stacking portion referred to as a saddle, a driver for driving staples and a clincher for bending staples driven into a bundle of sheets by the driver are required to be accurately positioned at a predetermined position. Conventionally, the bundle of sheets folded at the center portion thereof is aligned on the saddle, conveyed between the driver and the clincher, and then stapled. For this reason, the folding position of the bundle of sheets is not aligned with position of stapling, or it takes time to perform the positioning adjustment for the alignment.